Last verified 19 August 2026.
The formulas
| Formula | Use |
|---|---|
PREN = Cr + 3.3Mo + 16N | The one in general use |
PREN = Cr + 3.3(Mo + 0.5W) + 16N | Grades containing tungsten |
Elements as weight percentages. Nitrogen coefficients of 27 and 30 also appear in the literature; the BSSA notes that because nitrogen levels in most stainless steels are modest, the alternatives do not materially change how grades rank against each other.
Source: British Stainless Steel Association, calculation of pitting resistance equivalent numbers.
What it is actually for
PREN exists to answer one question: of these grades, which should resist chloride pitting better? It answers it well and cheaply, from a composition you already have.
The reason it works at all is that the three elements in the formula are the ones that do most of the work on the passive film. Chromium forms it, molybdenum stabilises it against chloride attack, nitrogen improves resistance and also strengthens the alloy. The coefficients are empirical weightings, fitted to observed behaviour — not derived from theory.
That last sentence is the whole of the caveat, and it deserves stating plainly.
What it cannot tell you
The BSSA puts it in italics, which for a trade association is close to shouting. PREN values are
“useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.”
Four specific things PREN does not account for:
Temperature. Pitting resistance falls with rising temperature and PREN has no temperature term, so two grades ranked correctly at room temperature can swap behaviour in warm service. The temperatures at which that happens are grade-specific and are not sourced on this page. The property that carries this is the critical pitting temperature — a measured quantity, obtained by test, not a calculation.
Crevice corrosion. The usual failure mode under gaskets, at flange faces, under deposits and at tube-to-tubesheet joints. Crevices initiate at lower severity than open-surface pitting, and PREN does not predict them. On a heat exchanger this is often the failure that actually happens.
Surface condition and fabrication. Weld heat-affected zones, heat tint, embedded iron, pickling and passivation state and surface finish all move real pitting resistance substantially. None of them are in the composition.
The actual heat. PREN is calculated from a composition. Which one — the specification limit, the nominal, or the analysis on your mill certificate? They differ, and the answer changes the number. See reading a mill certificate.
The evidence-level problem
A PREN quoted on a datasheet is E3 at best under the E-ladder, and the reason is instructive: it is a computed value, but the source does not state which formula it used or which composition it fed in. Computed without transparency is not E2 — E2 requires the method and the parameters, so a third party can repeat it.
That is not a technicality. Whether a grade shows PREN 34 or 36 can depend entirely on whether the supplier used specification minima, nominal composition or a typical heat analysis, and on which nitrogen coefficient they picked. If PREN is doing real work in your decision, calculate it yourself from a composition you can name, and record which one.
Calculated from a stated composition with the formula named, your PREN is E2. Calculated from your mill certificate’s actual analysis, it inherits from an E5 input — though under the weakest-link rule it stays E2, because a calculation cannot rise above the transparency of its own method.
Rough bands, and why they are rough
| PREN | Broadly |
|---|---|
| under 25 | not for chloride service |
| 25–32 | mild chloride exposure, low temperature |
| 32–40 | typical duplex territory |
| above 40 | super duplex and 6% Mo austenitics, seawater-capable given the right conditions |
Treat these as orientation, not as thresholds. A screening threshold of PREN ≥ 32 is a reasonable way to shorten a candidate list — it is what the worked example in the TRACE specification uses — and it is not a statement that anything above 32 will survive.
For reference: duplex 2205 is quoted at PREN 35 on the Outokumpu Forta datasheet (E3). 2205 against 316L sets that number in context.
How to use it without being misled
Do use it to shorten a list. That is what it is for, and it does it in seconds.
Do record which formula and which composition produced the number. A PREN with no stated basis is a number you cannot defend, and it is the kind of number that gets copied between projects until nobody remembers where it started.
Do not use it as a pass criterion for a corrosive service. Passing a PREN threshold is not a qualification. If the service is genuinely aggressive, what settles it is critical pitting temperature testing, crevice testing, or operating experience in that medium — measured things, not calculated ones.
Do not compare a PREN computed from specification minima with one computed from a typical analysis. That is the specified-versus-typical trap in a different costume, and it favours whichever grade happened to be quoted the generous way.
What this page does not cover
- Critical pitting temperature and critical crevice temperature, which are the measured properties PREN approximates. They deserve their own page.
- How to run a pitting test. ASTM G48 and its relatives govern; this page does not.
- Nickel alloys. PREN is used well outside stainless steels and the interpretation shifts.
- The derivation of the coefficients. They are empirical fits and this page does not trace their provenance.
- Whether any specific grade suits your service. That is the one question PREN cannot answer and this page will not pretend to either.